The binding of opiate ligands to the opiate receptor system is investigated in vivo in intact rat brain, in order to determine the relationships among opiate pharmacokinetics and metabolism, in vivo receptor binding and pharmacological effects. Current results demonstrate that there are significant qualitative and quantitative differences in opiate receptor binding in vivo and in vitro, including a partial loss of binding sensitivity to guanyl nucleotides which are thought to mediate receptor-effector coupling. Because of these in vivo - in vitro differences, new labeling techniques are being developed to study the opiate receptor multiplicity in vivo. At least three and possibly four distinct binding site populations can now be differentiated with our methods. Most strikingly, the in vivo labeling method has revealed the existence of novel type of binding sites with high selectivity for 4,5-epoxymorphinans (e.g., naloxone). These unique binding sites rapidly decay in vitro to a state of low affinity. Since receptor binding occurs in vivo, it can be correlated to the pharmacological effects; this approach may, therefore, yield information on the functions of individual receptor types. For example, results with buprenorphine suggest non-competitive auto-inhibition among the opiate receptor types. Recently, we have studied the interaction of a series of opiate agonists, mixed agonist antagonists, and antagonists to a type of in vivo binding site that resembles the Mu site characterized in vitro. This site appears to mediate the analgesic action of sufentanil and etorphine, but alone is insufficient to account for the action of mixed agonist-antagonists. Therefore, the proposed studies will include further development of the in vivo receptor labeling method and will address the question of the pharmacological functions of each type of opiate binding site. Moreover, changes of opiate receptor binding will be investigated that occur immediately after sacrifice of the aminals and homogenization of the brain tissues.